Autolite Engineer Provides Good Details on Spark Plugs

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Spark Plug Close

Lots of people like to share their information on spark plugs online, but many of them are wrong. Engine Labs cuts through the b.s.

Unless you race a vehicle with diesel power, the odds are good that at some point, you have had to select the appropriate spark plugs for your high-performance engine. If you weren’t sure which plugs were right for your build, you likely turned to some online resource for help, but with so much misinformation on spark plugs being shared online, there is a good chance what you think that you know is wrong.

To help address the lack of knowledge on spark plugs, the crew at Engine Labs sat down with Jerry Reeves, Director of Engineers for Autolite, to get the accurate low-down on the pulse of our engines.

Pre-Ignition vs. Detonation

The first information detailed in the original article looks at the differences between pre-ignition and detonation. Pre-ignition is generally caused by an ignition source in the cylinder, such as the ground strap on an extended-nose spark plug while detonation is caused without an ignition source – typically happening when internal cylinder temperatures get too high.

Detonation can be caused by a lean tune or crappy fuel while pre-ignition is often caused by using the wrong type of spark plug for your given application.

Heat Range

Some people believe that heat range varies by the style of electrode or the material used to construct the plug, but heat range is determined by the length the ceramic insulator around the electrode. A hot plug has more ceramic, so the spark has to travel further to escape the plug and that creates more heat, while a shorter ceramic sleeve leads to a cooler plug.

Two Spark Plugs

Cold plugs are generally better for high performance applications, as they produce less internal heat and they often have a shorter ground strap – leading to less occurrences of pre-ignition.

Fine-Wire Plugs

Some people believe that newer, fine-wire plugs tend to be hotter, but that is not the case. The size of the electrode has no measurable impact on the plugs heat range, but there is a performance advantage to the fine-wire design.

The thin, often iridium-coated electrodes require less voltage to create a spark, so running fine-wire plugs can produce less of a load on your ignition system. These thin wires are coated in precious metals to improve durability, but the Autolite exec stressed that this doesn’t cause a significant amount of extra heat in the engine. Most importantly, these plugs don’t get hot enough to cause pre-ignition.

Fine Wire Plugs

There is also the belief that a copper-core, thick-electrode plug will do a better job of burning off excessive gases and carbon deposits, but that is dependent purely on the heat range. The thickness of the electrode or what it is coated it has no impact on heat range, thus those factors have no impact on how well the plug burns off unwanted carbon and gases.

Which Plug for You?

If you are building an engine that will spend lots of time in routine driving situations, like idling in traffic or cruising at low engine speed, a hotter plug is a good idea. This will generally reduce engine emissions while also knocking out any excessive carbon in the combustion chamber. On the other hand, if you are building a high performance engine for racing that already generates a ton of heat, you don’t want to introduce any extra heat to the combustion process – making a cooler plug a good idea.

Group of Plugs

As the original article points out, the many of the differences from one plug to another might improve performance or emission levels, but things like the materials used and the length of the threads don’t have the impact on heat range that some people believe.

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